The present invention relates to an uninterruptible power supply. More particularly, the invention relates to a system in which several power sources are coupled to a common load to achieve uninterruptibility of the power supply.
U.S. Pat. No. 4,645,940 issued to the present inventor for Interrupt-Free, Unregulated Power Supply on Feb. 24, 1987. In this United States patent, FIG. 1 shows several salient pole alternators coupled to a common load via chokes. The present invention eliminates those chokes and simplifies the control scheme by utilizing the inherent controllable inductive reactive and output power as a function of slip characteristics of cast squirrel cage rotor asynchronous (induction) generators together with appropriate drive, excitation and control means.
Induction machine theory states that electrical input power will be converted to mechanical (shaft) output power when such a machine operates below synchronous speed by a specified slip fraction "s". The larger the load demand, within machine ratings, the larger "s" becomes. The machine is thus a "motor". The same theory avers that, if the shaft is driven above synchronous speed by a similar slip fraction "-s" while the stator is electrically excited, shaft input mechanical power will be converted to electrical output power at the stator terminals. The voltage and frequency of this power will be determined by the parameters of the excitation source alone. The heavier the load demand, the higher the absolute value of "-s" must be. The machine then functions as an asynchronous generator.
The connection of two or more such sources in parallel in the manner shown in FIGS. 1 and 2 herein, with appropriate control means, will result in an interrupt-free AC power supply. This type of system will perform as well as that described in U.S. Pat. No. 4,465,940 and will, in addition, provide better static frequency and voltage regulation and will have the capability to handle constant power loads.
Control of slip alone results in a 25 to 30% variation in output voltage phase angle over the load range of a standard design of machine. This is more than enough to achieve the necessary degree of uninterruptibility. Typically, voltage excursions of less than 10% of nominal, phase excursions of less than 10.degree. and recovery within less than 1/3 cycle are required. Control of the phase between the excitation voltage V.sub.exc and the load voltage V.sub.L further reduces the excursions and speeds recovery from transient disturbances.
A relatively broad range of load power factors may be accommodated as well, with no loss of control or stability. This statement is particularly important, for, given that system performance is primarily dependent on generator characteristics and the control thereof, both constant power and constant impedance loads are equally well served. This differs from the system of U.S. Pat. No. 4,645,940. In the system of the present invention, load current dynamics are easily accommodated without major load voltage disturbance by the very nature of the control technique, since a change in demand results in a change in slip "-s", which then causes a change in the rotor equivalent reactance (referred to the stator) -sX.sub.R of the asynchronous generator. In the power supply of U.S. Pat. No. 4,645,940, there is no change in coupling inductance, slowing the maximum current change rate, slowing system response, and creating a load voltage and phase transient of some magnitude depending on the design. The same reasoning clearly shows that the power supply of the present invention is much more capable of rapidly handling the switching of large blocks of load power on and off.
Improved static regulation occurs in the power supply of the invention, because generator terminal voltage depends only on excitation voltage levels, not on load current or power factor, or, for that matter, any other external cause/phenomenon.
Finally, this concept produces the lightest, most efficient interrupt-free power supply for any particular application, due to the facts that (a) asynchronous machines are lighter, smaller and more efficient than synchronous machines, (b) external chokes are not required and (c) the excitation and control power is lower for the system of the invention than an equivalent one using synchronous generators. Furthermore, since the system of the invention is simple and has few parts, reliability and maintainability are enhanced.
The principal object of the invention is to provide an uninterruptible power supply of simple structure wherein the power fraction of each of at least two sources may be set independently of the total load involved, so that failure of any source permits the load to be fed uninterruptedly.
An object of the invention is to provide an uninterruptible power supply having at least two sources, the power fraction of each of which may be set independently of the total load involved, so that failure of any source permits the load to be fed uninterruptedly and the fraction from each remaining source is adjusted to account for the number of failed sources.
Another object of the invention is to provide an uninterruptible power supply wherein failed ones of at least two sources are disconnected following failure in order to prevent a stationary machine from becoming a faulted or parasitic load on the power system.
Still another object of the invention is to provide an uninterruptible power supply suitable for use in aircraft, submersibles and aerospace vehicles, where space and weight are closely limited and regulation is necessary or desirable.
Yet another object of the invention is to provide an uninterruptible power supply which utilizes a multiplicity of rotary powered electrical sources and static backup emergency battery-energized inverter sources.
Another object of the invention is to provide an uninterruptible power supply in which either one source at a time will carry the full load, or in which several sources will share the required load power in accordance with a predetermined scheme.
Still another object of the invention is to provide an uninterruptible power supply of great reliability, minimum weight and minimum volume and which is inexpensive in maintenance.
Yet another object of the invention is to provide an uninterruptible power supply which eliminates coupling chokes and utilizes the controllable inductive reactance characteristics of asynchronous generators together with drive, excitation and control means.
Another object of the invention is to provide an uninterruptible power supply which serves both constant power and constant impedance loads equally well.
Still another object of the invention is to provide an uninterruptible power supply in which load current dynamics are easily accommodated without major load voltage or frequency disturbance, since a change in demand is accompanied by a change in the equivalent rotor reactance of the asynchronous generator.
Yet another object of the invention is to provide an uninterruptible power supply which is capable of readily responding to the switching of large blocks of load power on and off.
Another object of the invention is to provide an uninterruptible power supply having improved static regulation, due to the dependence of generator terminal voltage only on excitation voltage levels, not on load current, power factor, or any other external cause or phenomenon.
Still another object of the invention is to provide an uninterruptible power supply which is more efficient, more effective, lighter in weight, smaller in volume and lower in excitation and control power than the power supply of U.S. Pat. No. 4,645,940 and is of simple structure, great reliability and easy maintainability.